It is known in the art relating to inlet throttled supercharged engines to provide two variable flow orifices or throttles to control the load of the engine. Control electronics and calibrations are required for each orifice. Referring to FIG. 1, a typical prior art arrangement is represented by an engine 10 including a cylinder block 11 mounting an intake manifold having an air inlet 12 and an exhaust manifold having an exhaust outlet 13. The engine 11 drives a positive displacement charge compressor or supercharger 14 which delivers charge air through a charge cooler 15 to the inlet 12 of the intake manifold. The supercharger 14 receives charge airflow from a primary inlet throttle 16, typically a blade (or butterfly) type inlet throttle (recently Electronic Throttle Control or ETC). A smaller secondary blade type bypass throttle 17, controlled, for example, by a solenoid with a spring-over-diaphragm actuator, not shown, is connected between the outlet 18 of the charge cooler 15 and the inlet of the supercharger 14. An exhaust gas recirculation (EGR) valve 19 controls exhaust gas flow for emission reduction from the engine 11 exhaust outlet 13 to the charge compressor 14, downstream of the inlet throttle 16.
The bypass function is the inverse of the inlet throttle such that, as the inlet throttle 16 is opening, the bypass throttle 17 is closing. The bypass actuator typically relies on vacuum generated by the inlet throttle 16 to open the bypass throttle 17 and as such is nearly a binary control (either open or closed). As the bypass 17 closes, the pressure ratio across the supercharger rises rapidly with a rapid change in crankshaft torque with no change in inlet throttle area, making hardware and software calibration difficult to achieve a desirable sensitivity to driver pedal position. A simplified and easier to control throttle arrangement is desired.